Dear Dave Yes, I think you would have a multi-dimensional node_count. I think that fits the design, because a DSG can always be thought of as though it were an orthogonal array, albeit sparsely filled, with an instance dimension and an element dimension. The instance dimension is a discrete axis in the sense of CF section 4.5. Instead of this single discrete axis, you could have several spatial axes.
In fact, in the case I mentioned of a polyhedron, I imagine the data array might have to be 1D anyway, because you can't generally flatten a polyhedron into a rectangular 2D array of faces. So that would be formally just like the case of a DSG. Until/unless there is a use-case for the application of simple geometries outside DSGs we don't need to say anything about this, but I suggest that the possibility of its being needed means your new section doesn't belong in ch 9. Best wishes Jonathan ----- Forwarded message from David Blodgett <[email protected]> ----- > Date: Thu, 16 Mar 2017 14:19:43 -0500 > From: David Blodgett <[email protected]> > To: Jonathan Gregory <[email protected]> > CC: [email protected] > Subject: Re: [CF-metadata] Geometries in NetCDF Update > X-Mailer: Apple Mail (2.3259) > > Dear Jonathan, > > Thank you for looking things over. > > Makes sense that we would add a new ‘geometry’ attribute that would point at > the geometry container variable and be in both the node coordinates and the > data variables. > > Yes, the 0 or 1 is meant to be inside or outside. We thought that would be > specified in the document. I hadn’t thought of it as a true/false, but 0 and > 1 are nice because they can be treated that way. We could also use O and I, > but I’m hesitant to use characters, where an integer will suffice. We will > choose a suitable name that allows the variable to be interpreted as a > true/false. Your suggestion is a good starting point, I’ll see how Tim feels > about it. > > I spent a lot of brain cycles trying to figure out how a “count”ed contiguous > ragged array of nodes could be made to apply to a grid. Would you have a > multi-dimension count variable? I guess I’d need to see the cdl structure to > understand that case. I’m stuck on the “count” needing to be on the instance > dimension. > > Tim and I will reconcile some of this in the README as well as a more > detailed wiki writeup in that GitHub repository then submit the Trac ticket > with a proposed section for the spec unless others have input we should > consider? > > Regards, > > - Dave > > > On Mar 16, 2017, at 12:28 PM, Jonathan Gregory <[email protected]> > > wrote: > > > > Dear Dave > > > > Thanks for this > >> https://github.com/twhiteaker/netCDF-CF-simple-geometry/blob/master/README.md > > I think your explanation of what is needed and why is very clear, and I like > > the present form of the proposal. > > > > You asked me what I'd expect the bounds attribute to point to. This is my > > only > > reservation about the current design. We agree that the simple geometries > > are > > a kind of complicated bounds specification. However, we don't have to use > > the > > existing bounds att with a new purpose because of that. It currently points > > to > > a variable which must be numeric and have particular dimensions; hence one > > could tell reliably if it was a simple geometry instead if we insist that > > the > > geometry container variable must be a scalar, for example. So this does > > work, > > but to make it work would require changing existing software, which > > otherwise > > would think this new convention is an error. That wouldn't be friendly. > > > > Therefore I would suggest *not* using the bounds att of the aux coord vars, > > but > > instead giving them a new att e.g. geometry, to point to the geometry > > container > > variable. We can make a rule that it's an error to have both a geometry and > > a > > bounds att, like we do with climatology and bounds att. > > > > I'd also suggest requiring the data variable to have the same geometry att. > > That's because CF is generally "centred" on the data variable. If you've > > found > > the data variable, it's convenient to go straight from there to the geometry > > spec, rather than having to go via the aux coords. However I can see that > > you > > also want to attach it to the coordinates, since you might want to describe > > a > > geometry without a data variable at all but with nominal coordinates > > (although, > > at present, CF always expects there to be data variables). > > > > Arising from your discussion with Dan, I suppose you could say that if there > > is no part_node_count it must imply there is only one part per instance i.e. > > you can omit this attribute if it's not needed. > > > > I assume that 0 means inside and 1 outside for the polygon_ring_type. If > > it's > > a binary choice like that, maybe it would be more self-explanatory to call > > it > > outside, or something else which indicates the convention. > > > > I would note that this new convention is applicable for data on a grid too > > i.e. on a polyhedron which is composed of more than one type of polygon. Its > > usefulness is not limited to DSGs. > > > > Best wishes > > > > Jonathan > > _______________________________________________ > > CF-metadata mailing list > > [email protected] > > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > > > ----- End forwarded message ----- _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
